Agents with the Internal Revenue Service, Drug Enforcement Agency and U.S. Marshals were spotted outside the medical marijuana facility, with some entering the site with power saws and a sledgehammer and other carrying out bags of material to a van, nbcbayarea.com and the San Francisco Chronicle reported.

Two other buildings in the city’s cannabis district -- known as Oaksterdam -- were also raided.

Calls and emails seeking comment from the IRS, DEA and Oaksterdam were not immediately returned. But IRS spokeswoman Arlette Lee told local media that the probe was under seal in a Northern District Court and the agents were serving a federal search warrant.

Federal prosecutors have recently been stepping up pressure on medical cannabis dispensaries, forcing hundreds to close, the Chronicle reported. They maintain that federal law trumps California's 1996 voter-approved law legalizing medical cannabis.

The raid comes about two weeks after the Oakland City Council approved doubling the number of medical marijuana dispensaries, approving four new potential operators, the Oakland Tribune reported.

Many view Oaksterdam as the heart of California's pot legalization movement. In business since 2007, Oaksterdam was founded by Richard Lee -- the main backer of the 2010 state ballot measure that would have legalized marijuana for recreational use if it had passed, nbcbayarea.com reported.

Oaksterdam University teaches classes on how to cultivate marijuana plants for personal use.